1. Field of Invention
The present invention relates to an operation of image-sensing unit and image-sensing device using the same. More particularly, the present invention relates to an operation of image-sensing unit and image-sensing device using the same capable of increasing the dynamic range of the image-sensing units.
2. Description of Related Art
There are more and more electronic products with built-in camera functions, such as mobile phones, personal digital assistants (PDAs) and toys. With the rapid development of electronic technology, the image sensors have gradually replacing the traditional films as major image sensing elements. The purpose of the image sensors is to convert light signals to electronic signals. A lot of the image sensors available in the market now use built-in photodiodes for acquiring light signals.
FIG. 1 shows a circuit diagram of a conventional image sensor. Please refer to FIG. 1. The image sensor 100 comprises a reference voltage Vcc, a photodiode 120, a first switch 130, a source follower 140, a second switch 180 and a memory circuit 160. The first switch 130, the source follower 140, and the second switch 180 can be transistors. The photodiode 120 and the source follower 140 are both electrically coupled to the first switch 130, and the diode 120 and the source follower 140 are both electrically coupled to the reference voltage Vcc. The first switch 130 is disposed between the diode 120 and the reference voltage Vcc. Besides, the gate of the source follower 140 is electrically coupled between the first switch 130 and the photodiode 120. The memory circuit 160 of the image sensor is used to record the variation of the output voltage Vout of the second switch 180, which is proportional to the voltage value of the gate of the source follower 140. The operation procedure of the image sensor 100 is discussed in detail below.
FIG. 2 schematically shows the variation of the output voltage of FIG. 1 in an operation cycle of an image sensor. Please refer to FIG. 1 and FIG. 2. As the operation cycle begins, the switch 130 is turned on. The voltage V1 of the photodiode 120 and the voltage value of the source follower 140 will be equal to the reference voltage Vcc. The switch 130 is then turned off at a first time T2, and the outer light 150 irradiates the photodiode 120 via lenses (not shown). Due to illumination of the light 150, the photo current is generated by the photodiode 120 and therefore the voltage V1 of the photodiode is lowered. Consequently, the voltage of the gate of source follower 140 reduces, too. Meanwhile, the output voltage Vout changes according to the voltage variation of the gate of source follower 140. Later, at the second time T2, again the first switch is turned on to start a new cycle. The output voltage of first time T1 and that of the second time T2 are recorded by the memory circuit 160, and by altering the difference in between, the image sensor can determine the intensity of outer light 150.
Please refer to FIG. 1 and FIG. 2. It can be found that, the more intense the outer light 150 is, the faster the output voltage Vout decreases. When the output voltage Vout drops to zero before the second time T2, the image sensor 100 is unable to determine the intensity of outer light 150. Therefore, there is a limit of the dynamic range for the image sensor 100 (Dynamic Range=the maximum intensity of light detectable by the image sensor/the minimum intensity of light detectable by the image sensor).